After running OpenSim on mono 3.12 for a while my recommendation is to increase the number of allowed open files mainly to prevent a crash during startup.
My current recommendation is to allow for a max of 4096 open files per process. Use the following command in terminal from a privileged account to set this.
sudo launchctl limit maxfiles 4096 65536
Make sure to log out and back in the account OpenSim is running under before (re)starting it. You can check allowed open files with the command ulimit -a
The increased limit is because a large number of cache and other files are opened/created during the startup phase of OpenSim. You will typically see this when generating map tiles, generating physics for in-world prims and meshes, and when compiling scripts.
NOTE: The above command sets the allowed open files at the system level, which also sets it for terminal logged-in sessions as can be seen by the ulimit -a command. If you for some reason need to increase the number of open files for applications running on your OS X desktop, you have to increase the number by using the launchctl limit maxfiles 4096 65536 command in a terminal session for that logged in user, or a suitable plist placed in ~/Library/LaunchAgents to make it persistent across logins. A non-privileged user cannot set the limits higher than the max system limits.